C++ lecture 17

C++ Programming
Lecture 17

Pointers
– Part
II
The Hashemite
University
Computer Engineering
Department
(Adapted from the textbook slides)


Outline
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Introduction.
The Relationship Between Pointers and
Arrays.
Pointer Expressions and Pointer Arithmetic.
sizeof Operator for Arrays and Pointers.
Functions call by reference using pointers.
Examples.

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The Relationship Between
Pointers and Arrays I
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Arrays and pointers are closely related
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Array name is a constant pointer that contains the
address of the first element in the array (you cant
move this pointer to points to another element of
the array).
Pointers can do array subscripting operations
Having declared an array b[ 5 ] and a pointer bPtr
You have two ways to make bPtr points to the
first element of the array
1 -

bPtr is equal to b
bptr == b

2 - bptr is equal to the address of the first element of b
bptr == &b[ 0 ]

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The Relationship Between
Pointers and Arrays II
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Accessing array elements with pointers
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Element b[ n ] can be accessed by *( bPtr + n )
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Array itself can use pointer arithmetic.
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Called pointer/offset notation
b[ 3 ] same as *(b + 3)
Note : *(b+3) is correct, while *(b+=3) is not
correct because you try to move this pointer to
another location

Pointers can be subscripted (pointer/subscript

notation)
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bPtr[ 3 ] same as b[ 3 ]
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Example --Array name is pointer
to first element
char t[5]={'a','b','c','d','e'};
cout<< *t<*t=‘w';
cout<
"<<*t; // will print w w

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Pointer to Char and Strings
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Character array:
char color[] = "blue";

Creates 5 element char array, color, (last element is

'\0'), the same effect of:
char color[] = {‘b’, ‘l’, ‘u’, ‘e’, ‘\0’};
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variable of type char *
char *colorPtr = "blue";
cout<< *colorPtr; // will print b
cout<< colorPtr; // will print blue
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Creates a pointer to string “blue”, colorPtr, and
stores it somewhere in memory
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Pointer Expressions and
Pointer Arithmetic I
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Pointer arithmetic
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Increment/decrement pointer (++ or --)
Add/subtract an integer to/from a pointer( + or += , - or
-=)
Pointers
may be subtracted from each other
 
Pointer arithmetic is meaningless unless performed on an
array

5 element int array on a machine using 4 byte
location
ints
3000
3004
3008
3012
3016
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vPtr points to first element v[ 0 ], which is at location
v[0]
v[1]
v[2]
v[3]
3000
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vPtr = 3000

vPtr += 2; sets vPtr to 3008
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v[4]

vPtr points to v[ 2 ]

pointer variable vPtr

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Example
address
es
double array[5]

293
8

294
6

2.3 -5

295
4

-1

296
2

297
0

3.3 4.9

double *pointer = &array; //  pointer = 2938 points to
array[0]
pointer++; //  pointer = 2946 points to array[1]
pointer+=2//  pointer = 2962 points to array[3]
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Pointer Expressions and
Pointer Arithmetic II
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Subtracting pointers
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Returns the number of elements between two addresses

vPtr2 = &v[ 2 ];
vPtr = &v[ 0 ];
vPtr2 - vPtr = 2

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Pointer comparison
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Test which pointer points to the higher numbered array
element
Using the above pointers
if(vptr > vptr2)

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Test if a pointer points to 0 (NULL)
if ( vPtr == NULL )
statement
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sizeof Operator and
Arrays
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sizeof
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Returns size of operand in bytes
Returns the result of type size_t which is unsigned
integer.
For arrays, sizeof returns
( the size of 1 element ) * ( number of elements )

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if sizeof( int ) = 4, then
int myArray[10];
cout << sizeof(myArray);

will print 40
 To get the size of an array (number of elements) using
sizeof operator do the following:
Array size = sizeof(myArray)/ sizeof(int);
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sizeof Operator and
Pointers
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Applying sizeof operator for a pointer always returns a

result of 4 regardless of the data type to which the
pointer is pointing.
char x;
char *p=&x;
double y=0;
double *ypt=&y;
cout<cout<points)
cout<cout<points)
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Calling Functions by
Reference
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Call by reference with pointer arguments
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Pass address of argument using & operator
Allows you to change actual location in memory

Arrays are not passed with & because the array name is
already a pointer contains the address of the first element of
the array in the memory.
* operator used as alias/nickname for variable inside of
function
void doubleNum( int *number )
{
*number = 2 * ( *number );
}

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*number used as nickname for the variable passed in
When the function is called, must be passed an address
doubleNum( &myNum );
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// Fig. 5.7: fig05_07.cpp
// Cube a variable using call-by-reference
// with a pointer argument
#include <iostream>
using std::cout;
using std::endl;
void cubeByReference( int * );

//

Notice how the address of
number is given cubeByReference expects a
prototype
pointer (an address of a variable).

int main()
{
int number = 5;

}

cout << "The original value of number is " << number;
cubeByReference( &number );
cout << "\nThe new value of number is " << number << endl;
return 0;
Inside cubeByReference,

void cubeByReference( int *nPtr )
{
*nPtr = *nPtr * *nPtr * *nPtr;
}

*nPtr is used (*nPtr is
number).
// cube number in main

The original value of number is 5
The new value of number is 125

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Example I
// Converting lowercase letters to uppercase letters
// using a non-constant pointer to non-constant data.
#include <iostream.h>
#include <cctype> // prototypes for islower and toupper
void convertToUppercase( char * );
int main()
{
char phrase[] = "characters and $32.98";
cout << "The phrase before conversion is: " << phrase;
convertToUppercase( phrase );
cout << "\nThe phrase after conversion is: “ << phrase <<
endl;
return 0;
} // end main

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Example I … cont.
// convert string to uppercase letters
void convertToUppercase( char *sPtr )
{
while ( *sPtr != '\0' ) { // current character is not '\0'
if ( islower( *sPtr ) ) // if character is lowercase,
*sPtr = toupper( *sPtr ); // convert to uppercase
++sPtr; // move sPtr to next character in string
} // end while

} // end function convertToUppercase

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Example II
// Copying a string using array notation and pointer notation.
#include <iostream.h>
void copy1( char *, const char * ); // prototype
void copy2( char *, const char * ); // prototype
int main()
{
char string1[ 10 ];
char *string2 = "Hello";
char string3[ 10 ];
char string4[] = "Good Bye";
copy1( string1, string2 );
cout << "string1 = " << string1 << endl;
copy2( string3, string4 );
cout << "string3 = " << string3 << endl;
return 0; // indicates successful termination
} // end main

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Example II … cont.
// copy s2 to s1 using array notation
void copy1( char *s1, const char *s2 )
{
for ( int i = 0; ( s1[ i ] = s2[ i ] ) != '\0'; i++ )
; // do nothing in body
} // end function copy1
// copy s2 to s1 using pointer notation
void copy2( char *s1, const char *s2 )
{
for ( ; ( *s1 = *s2 ) != '\0'; s1++, s2++ )
; // do nothing in body
} // end function copy2
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Additional Notes
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This lecture covers the following material from the textbook:
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Chapter 5: Sections 5.6 – 5.8

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